1. Field of the Invention
The present invention relates to a data reception device for receiving a video signal and an audio signal in MPEG2-TS or the like, which are transmitted via a wired line or a wireless line, and more particularly, relates to a data reception device which is capable of ensuring synchronization of transmitted video signals and audio signals or the like.
2. Description of the Related Art
Conventionally, in order to transmit a video signal, an audio signal, or the like via a wireless line such as a wireless local area network, which is typically in accordance with IEEE802.11, there has existed a technique for realizing synchronization of signals between a transmitting side and a receiving side (for example, refer to a patent document 1).
FIG. 11 is a block diagram illustrating a functional structure of the conventional wireless master device 900, described in patent document 1 (Japanese Laid-Open Publication No. 2003-60652), on a transmitting side. FIG. 12 is a block diagram illustrating a functional structure of the conventional wireless slave device 920, described in the patent document 1, on a receiving side. FIG. 13 is a diagram showing a frame structure of the conventional data packet 990, described in the patent document 1, transmitted from a wireless mater station. Hereinafter, with reference to FIG. 11, FIG. 12, and FIG. 13, operations of the wireless mater device 900 and the wireless slave device 920 in the patent document 1 will be described.
On the wireless master device 900, a video signal inputted to an MPEG2-TS encoding unit 901 is converted to an MPEG2-TS packet 950 and outputted. A time stamp addition unit 902 adds to the MPEG2-TS packet 950a time stamp 951 in accordance with time at which the MPEG2-TS packet 950 is outputted from the MPEG2-TS encoding unit 901. The time stamp 951 added here contains a timer value from a beacon timer unit 908.
A communication protocol processing unit 903 holds together, as one data block 941, the MPEG2-TS packet 950 with the time stamp 951 added, combines a data block 941 and a plurality of data blocks 942 to 94N, adds a data header 980 therein, and outputs a resultant. A frame assembling unit 909 in an MAC processing unit 904 adds an MAC header 970 to the resultant outputted from the communication protocol processing unit 903 and outputs a resultant. An RF unit 905 adds a physical layer header 960 to the resultant outputted from the frame assembling unit 909 and outputs a resultant. An antenna 906 emits the resultant outputted from the RF unit 905 as a wireless signal into a wireless communication line.
A clock generation unit 907 generates and inputs a clock to the beacon timer unit 908. Based on the clock, the beacon timer unit 908 sets and outputs a current timer value (for example, current time). A beacon generation unit 910 generates a beacon containing the timer value outputted from the beacon timer unit 908. The frame assembling unit 909 adds a MAC header to the beacon generated by the beacon generation unit 910 and outputs a resultant. The RF unit 905 adds the physical layer header 960 to the resultant outputted from the frame assembling unit 909. The antenna 906 emits the resultant outputted from the RF unit 905 as a beacon signal into the wireless communication line.
On the wireless slave device 920, the wireless signal transmitted via the wireless communication line is received by an antenna 921. The wireless signal received by the antenna 921 is inputted via an RF unit 922 to a frame analysis (analyzing) unit 925 in an MAC processing unit 923. The frame analysis unit 925 analyzes whether or not a destination of the received signal is the wireless slave device 920. When the destination is the wireless slave device 920, the frame analysis unit 925 inputs the received signal to a receive data processing unit 927. When the received signal is a data packet 990, the receive data processing unit 927 inputs the data packet 990 to a communication protocol processing unit 924. On the other hand, the received signal is a beacon signal, when the receive data processing unit 927 inputs the beacon signal to a beacon timer unit 926.
The communication protocol processing unit 924 analyzes a physical layer header 960, a MAC header 970, and a data header 980 in the received data packet 990, extracts data blocks 941 to 94N, and stores the extracted data blocks 941 to 94N in a receive data buffer unit 933.
Based on a clock outputted from a clock generation unit 928, the beacon timer unit 926 sets a current timer value (for example, current time). When the beacon signal is inputted from the receive data processing unit 927, the beacon timer unit 926 updates the current timer value so that the current timer value and a timer value contained in the beacon signal match with each other. Thereby the beacon timer unit 908 on the wireless master device 900 synchronizes with the beacon timer unit 926 on the wireless slave device 920.
A PLL processing unit 929 performs PLL processing for the timer value from the beacon timer unit 926 and inputs the processed timer value to a timer unit 930. The timer unit 930 inputs the current timer value, for which the PLL processing has been performed, to a time stamp comparison unit 931. A time stamp extraction unit 932 extracts a time stamp contained in a data block which has been stored in the receive data buffer unit 933. The time stamp comparison unit 931 determines whether or not the timer value contained in the time stamp which the time stamp extraction unit 932 has extracted and the current timer value inputted from the timer unit 930 match with each other. When a timer value contained in the time stamp matches with an ever-changing current timer value, the time stamp comparison unit 931 outputs to the receive data buffer unit 933 an MPEG2-TS packet in the data block containing the time stamp whose timer value matches with an ever-changing current timer value. An MPEG2-TS decoding unit 934 decodes the MPEG2-TS packet outputted from the receive data buffer unit 933 and outputs a video signal.
Thus, the wireless slave device 920 is capable of coinciding a time interval between MPEG2-TS packets inputted to the MPEG2-TS decoding unit with a time interval between MPEG2-TS packets outputted from the MPEG2-TS encoding unit 901 on the wireless master device 900.
Incidentally, the MPEG2-TS packet contains system clock information (PCR: Program Clock Reference) for the MPEG2-TS decoding unit 901. Based on this system clock information, the MPEG2-TS decoding unit 934 reproduces a system clock for the MPEG2-TS encoding unit 901.
In order to reproduce the system clock, however, a fluctuation in transmission time during which respective MPEG2-TS packets are transmitted must be sufficiently small. In ISO/IEC13818 Part 9 (MPEG2 Extension for real time interface), a value of the fluctuation in the transmission time is specified as 25 microseconds or less.
As described above, in the conventional data transmission system, the wireless master device 900 on the transmitting side adds to an MPEG2-TS packet a time stamp based on a beacon timer value. The wireless slave device 920 on the receiving side, using the beacon timer value which has matched with that of the wireless master device 900, controls timing of outputting an MPEG2-TS packet based on the time stamp of the MPEG2-TS packet. Therefore, it is possible to compensate a gap, on time axis, occurring when a plurality of MPEG-TS packets are combined to a packet to be wirelessly transmitted and a fluctuation in transmission time occurring on a transmission line. Therefore on the receiving side, an original time interval between MPEG2-TS packets can be reproduced, enabling synchronization of video signals and audio signals transmitted between the transmitting side and the receiving side.